Inferior vena cava (IVC) filters are devices that block the passage of venous emboli from the lower extremities or pelvis through the IVC into the pulmonary arteries. Generally, IVC filters are used to capture potentially fatal pulmonary emboli at an anatomical location where they may pose a minimal risk for the patient. Since the vast majority of pulmonary emboli originate from the lower body parts, filters are mainly placed into the IVC, and only very rarely is there an indication for filter placement in the superior vena cava (SVC).
Numerous filters are available for percutaneous placement in a patient. However, most filters are generally retrievable only up to about 14 days from deployment, because of fibrotic wall reactions and endothelialization that starts on the third week following deployment. Filters that remain longer than 2 weeks generally demonstrate an intimal coverage of the struts that were in contact with the vessel wall. Therefore, any manipulation of these implanted struts after the second week may result in grave injury of the vessel wall with subsequent complications including: life threatening bleeding, dissection of vessel and thrombosis. For this reason, venous filters are generally not used if not absolutely necessary.
A recent publication has demonstrated retrievability up to 134 days (Asch, Radiology Vol. 225 (no. 3), pp. 835-844, December 2002). That study used a recovery nitinol filter (RNF) (NMT Medical, Boston, Mass.), which is composed of 12 0.13 inch nitinol wires that extend from a nitinol sleeve. The filter was removed using a retrieval cone that docked with the filter tip. One possible disadvantage of the filter utilized in Asch is that the anchors must be pulled from the wall and this always creates the possibility of damaging the vessel wall.
Lifetime filters can be utilized in instances where the filter is necessary, but the complications of temporary filters are unacceptable. However, lifetime filters also have risks and complications, including: migration of the filter to the heart or lung, fracture of the filter legs, penetration of the IVC by filter components, thrombosis of the vena-cava, and an increased incidence of lower extremity deep vein thrombosis. Such long term indwelling filters are also associated with a high rate of vena cava clot or venous insufficiency symptoms from the inability of the blood to return to the heart in a hemodynamically efficient manner. In such instances, collateral veins develop, but can generally not handle the high flows around an occlusive vena cava clot surrounding a chronic filter. This can result in massive swelling of the lower extremities, pain and markedly dilated lower extremity veins. Such symptoms may occur for the lifetime of the patient and may be debilitating.
Because of the problems of removing temporary IVC filters, and the side effects of lifetime filters, there remains a need for a temporary filter that can be removed with a decreased amount or severity of complications.